1. Field of the Invention
The present invention relates to a die for forming a honeycomb structure for use in extrusion of a honeycomb structure.
2. Description of Related Art
In recent years, a honeycomb structure made of a ceramic superior in resistances to heat and corrosion has been utilized in various fields such as a diesel particulate filter (DPF) for capturing particulates discharged from a diesel engine, and a catalyst carrier for carrying a catalyst which purifies automobile exhaust gas.
In general, a honeycomb structure (formed article) obtained by extrusion is dried and fired to thereby manufacture the honeycomb structure. A die for use in extrusion of the honeycomb structure has a structure in which groovy slits are disposed by a plurality of cell blocks in the front surface of a matrix made of stainless steel, iron or the like, and back holes communicating with the slits are disposed in the back surface. When a forming raw material (clay) introduced from the back holes is extruded from the slits, the formed article having a honeycomb structure is obtained. That is, the clay passed and extruded through the slits in the die forms partition walls which surround each cell (through hole), and the cell blocks block a flow of the clay to form hollow portions which are the cells.
Additionally, in the conventional die for forming the honeycomb structure, pitches of the adjacent cell blocks are all constant, but in the honeycomb structure extruded using such a die and obtained through drying and firing, pitches of the cells (cell pitches) are not constant. There is a case where the cell pitch in the vicinity of an outer periphery is smaller than that of a central portion. This is supposedly because the cell pitches change with the central portion and the vicinity of the outer periphery owing to occurrence of deformation/distortion by non-uniform heating at a drying time or the like. When the cell pitch in the vicinity of the outer periphery decreases, an outer dimension of the whole honeycomb structure decreases, and sometimes deviates from standards of a product (a prior-art document concerning such conventional situation is not found).